1. Field of the Invention
The present invention relates to a semiconductor device and particularly to a semiconductor device having a superlattice structure.
2. Description of the Related Art
A high electron mobility transistor (hereinafter referred to as HEMT) has an electron moving region, which is almost two-dimensional and spatially separated from an impurity doping region. According to this structure, electron scattering caused by impurities is suppressed, thus increasing electron mobility in a low temperature and allowing HEMT to operate in a low noise level and in a high speed. However, in such an existing HEMT as described above, it is optical phonon scattering rather than impurity scattering that is dominant as electron scattering in the room temperature. Due to this reason, a HEMT having high electron mobility in the room temperature has not been developed yet.
In order to increase the electron mobility by suppressing impurity scattering, it is necessary to further improve the purity of a crystal. To this end, a very great effort is required to prevent unintended impurity from mixing in. Therefore, it is impossible in fact to attain an electron mobility, which is higher than that of the present one, by suppressing impurity scattering.